This invention relates generally to rod end bearing assemblies for use in high temperature, high altitude applications. In particular, the invention relates to rod end bearings for use in gas turbine engines.
In a gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The compressor is made up of several rows or stages of compressor stator vanes and corresponding rows or stages of compressor rotor blades therebetween. The stator vane rows are situated between the rotor blade rows and direct airflow toward downstream rotor blades on the rotor blade row. After leaving the compressor, the air/fuel mixture is combusted, and the resulting hot combustion gases are passed through the turbine section of the engine. The flow of hot combustion gases turn the turbine by contacting an airfoil portion of the turbine blade, which in turn rotates the shaft and provides power to the compressor. The hot exhaust gases exit from the rear of the engine, driving the engine forward. Optionally, a bypass fan driven by a shaft extending from the turbine section, which forces air around the center core of the engine and provides additional thrust to the engine.
Rod end bearings are bearing structures that connect together two rods, components or assemblies with a spherical or ball type joint. One type of rod known in the art, as described in U.S. Pat. No. 6,352,368, is a rod end bearing having an outer shell and having a spherical inner surface. Inside the outer shell there is a ball part. Between the ball part and the outer shell there is a bearing race. The outer shell and ball are fabricated from steel and the bearing is fabricated from polyurethane. Alternatively, rod end bearings having a molded race situated between the outer shell part and the inner ball part fabricated from injection molded reinforced nylon.
A number of structures in the gas turbine engine utilize rod end bearings. The rod end bearings include rod end bearings connecting the actuator mechanism to the variable stator vane. Additional structures in the gas turbine engine are subject to wear, including variable geometry exhaust actuating mechanisms. Each of the rod end bearing applications is subjected to conditions of wear at temperatures ranging from low temperatures to highly elevated temperatures. In addition, the rod end bearings are subject to high altitude atmospheres. In addition to low temperatures, high altitude atmosphere includes little or no water vapor. Water vapor is required for conventional graphite containing lubricants to maintain lubricity.
Wear occurs when contacting surfaces of two components rub against each other. Typical results from wear include scoring of one or both surfaces, and/or material removal from one or both surfaces. In rod end bearings used in the gas turbine engine, scoring may occur on one or both of the surface of the ball and the surface of the bearing casing, both of which are expensive to repair and/or replace. As the surfaces are damaged, they become even more susceptible to the effects of wear as their effective coefficients of friction rise and wear increases the clearance between the wearing surfaces, so that loads are more concentrated and causes undue motions, so the wear damage accelerates with increasing time in service. Wear debris, which may include material removed from the wearing surfaces due to wear, or may include foreign particles, such as dust or debris from the air traveling through the engine can further accelerate wear. In addition to the damage to the ball and the bearing casing, the coefficient of friction between the ball and the bearing casing increase, thereby increasing the amount of force required to move the rod and bearing. Therefore, as the rod end bearing wears, increased force is needed from an attached actuator to move the bearing during operation. The increased force requirement causes additional strain on the actuator and/or results in the need for a larger actuator.
To increase the operating capacity of the compressor, at least some of the compressor stator vane rows are designed with vanes that can rotate around an axis that is in its longitudinal direction to adjust the angular orientation of the vane with respect to the airflow traveling through the compressor. The adjustment of the angular orientation allows control of the amount of air flowing through the compressor. Variable stator vane designs typically allow for about 45° rotation of the stator vane to optimize compressor performance over the operating envelope of a gas turbine engine. The variable stator vane structures include an outer trunnion disposed in a complementary mounting boss in the stator casing for allowing rotation of the vane relative to the casing. A lever arm is fixedly joined to a coaxial stem extending outwardly from the vane trunnion. The distal end of the lever arm is operatively joined to an actuation ring that controls the angle of the vane. All of the vane lever arms in a single row are joined to a common actuation ring for ensuring that all of the variable vanes are positioned relative to the airflow in the compressor stage at the same angular orientation.
When a rod end bearing fails due to excessive wear, malfunctions in the gas turbine engine compressor may occur. The failure of the rod end bearing may create an increase in force required by the actuator to move the bearing and/or the attached system. In addition, the failure may result in a loss of control of the system connected to the bearing.
One known material for fabrication of wear surfaces for variable stator vane assemblies is a specially developed composite of carbon fiber reinforcing rods in a polyimide resin matrix manufactured by E. I. Du Pont De Nemours and Company of Wilmington, Del. The material is commonly known as VESPEL® CP™. VESPEL® and CP™ are trademarks that are owned by E. I. DuPont DeNemours and Company. The polyimide resin used in VESPEL® CP™ is commonly known as NR150™. The NR150™ trademark is owned by Cytec Technology Group of Wilmington, Del. Although the VESPEL® CP™ material has an extended life at temperatures 450-500° F. (232-260° C.), the VESPEL® CP™ bushing have an upper temperature limit of 600° F. (316° C.). Extended operation at temperatures at or above 600° F. (316° C.) limit their operational life. The material does not withstand the combinations of high temperature and vibrational loading experienced in the operation of the gas turbine engine well, leading to a relatively short part life.
Another known method for reducing wear on the variable stator vane assembly is placing a carbon-containing antifriction coating on sliding surfaces within the variable stator vane assembly. This antifriction coating is a coating fabricated from a material that reduces the coefficient of friction between the ball bearing and the bearing casing. One carbon-containing component known for antifriction coating is graphite. However, graphite has the disadvantage that water vapor is required to maintain lubricity. Atmospheres at aircraft cruise altitudes do not have enough water vapor present for graphite to be lubricious. Graphite also has the disadvantage in that graphite has poor tribological properties in applications that require reciprocating motion. An additional disadvantage of graphite is that graphite begins to oxidize rapidly at temperatures at or greater than 500° C. (932° F.). Some variable stator vane systems may experience temperatures in excess of 500° C. (932° F.). Therefore, a replacement material for graphite in antifriction coating is needed.
There is accordingly a need for an improved approach to the protection of gas turbine components, such as variable vane trunnion surfaces, variable vane casing surface or other surfaces in the gas turbine engine against the damage caused by wear. The present invention fulfills this need, and further provides related advantages.